254 PHYSICAL BASIS OF HEREDITY 



genes carried by the sex-chromosome only one can exist 

 at a time in the sex that has only one of these chromosomes. 

 In the indiAT-dual with two mutant allelomorphs one of 

 them replaces the normal allelomorph of the ordinary 

 Mendelian pair. The two mutant allelomorphs behave 

 towards each other in the same way as does the normal 

 towards its mutant allelomorphs. It is doubtful whether 

 we can conclude anything more from this relation of Men- 

 delian pairs than we knew before,^ although there is at 

 least a sentimental satisfaction in knowing that both nor- 

 mal allelomorphs can be replaced by mutant ones without 

 altering the working of the machinery. 



The linkage relation of each member of a series of 

 multiple allelomorphs to all other genes of its chromo- 

 some is, of course, the same. While the theory of identical 

 loci requires this as a primary condition it is not legiti- 

 mate to use this evidence as a proof of the identity of the 

 loci, because it is not possible to work with sufScient pre- 

 cision in locating genes by their relation to other linked 

 genes to distinguish between identical loci and close- 

 linked genes. 



The question of lethal genes has attracted in recent 

 years increasing attention, both on account of their fre- 

 quency and because of a curious complication they may 

 produce in hiding the effects of other genes also present. 

 In DrosophUa we have records of more than 20 sex-Unked 

 lethals, and about 15 not sex-linked, and scattering records 

 of many others. Gametic lethal genes are those that 

 destroy eggs or pollen cells that contain such genes. 

 Zygotic lethal genes affect the embryo, the larva, or the 

 adult, so that it dies. In the case of the garden plant 

 known as double "stocks," the genetic evidence obtained 

 by Miss Saunders indicates that certain kinds of pollen 

 are not produced, and presumably die because of a con- 

 tained factor. The same factor does not kill the ovules, 



'The substitution by crossing over really furnisbea as good a demon- 

 stration of this point. 



